The present invention relates to the magnetic resonance imaging arts. It finds particular application in conjunction with gradient coils for "short" bore magnets and will be described with particular reference thereto. It is to be appreciated, however, that the present invention will also find application in conjunction with coils for other magnets, particularly magnets in which a gradient coil is disposed in a magnetic field such that Lorentzian force components are not balanced.
Magnetic resonance imagers commonly include a bore having a diameter of 90 centimeters or more for receiving the body of an examined patient. The bore is surrounded by a series of annular superconducting magnets for creating a substantially uniform magnetic field longitudinally along the patient receiving bore. The more axially spaced the annular magnets, the more uniform the primary magnetic field within the patient receiving bore tends to be and the longer the axial dimension over which such magnetic field uniformity exists. Typically, the bore is at least 1.6 meters long and often longer.
One of the drawbacks to such "long" bore magnets is that the region of interest is often inaccessible to medical personnel. If a procedure is to be performed based on the image, the patient must be removed from the bore before the procedure can be performed. Moving the patient risks potential misregistration problems between the image and the patient.
One way to improve access to the patient is to shorten the length of the magnet and the patient receiving bore. If the magnet and the bore were shortened to about 1 meter or roughly the diameter of the bore, patient access is much improved. Although the size of the uniform magnetic field area compresses to a more disk-like shape, the area of substantial uniformity is still sufficient for a series of 10 to 20 contiguous slice images. NMR helical or continuous scanning methods can also be employed.
Although an adequate imaging volume remains, the magnetic field in the volume around the periphery of the bore which receives the gradient coil tends to become relatively non-uniform and has both axial z-components and radial x,y-components. The gradient coil generally includes windings for generating three linear and orthogonal magnetic field gradients for providing spatial resolution and discrimination of nuclear magnetic resonance signals. Gradient coils are typically designed and constructed to optimize strength and linearity over the imaging volume and stored energy and inductance in the gradient coil. See, for example, U.S. Pat. No. 5,296,810 of Morich. To create the magnetic field gradients, current pulses are applied to the x, y, and/or z-gradient coils. These currents interact with the main magnetic field to generate Lorentz forces on the gradient coil. Due to the symmetries included in gradient coil currents, the Lorentz forces across the entire coil cancel when the gradient coil is disposed in a uniform magnetic field. However, when the main magnetic field is less uniform, particularly when there are significant radial and non-uniform axial components in the neighborhood of the gradient coils, a net thrust can be developed. Typically, pulsing the z-gradient coil causes a net thrust in the z-direction, pulsing the x-gradient coil develops a net thrust in the x-direction, and pulsing the y-gradient coil causes a net thrust in the y-direction. In the case of the z-gradient coil, the net axial force can be on the order of a few hundred pounds. These net thrusts tend to push or urge the gradient coil axially out of the bore. Although the gradient coil can be anchored mechanically, these large forces still tend to cause acoustic noise and increased vibrations to the magnet and the patient. Such vibration has deleterious effects on imaging, such as a loss of resolution.
The present invention contemplates a new and improved gradient coil which overcomes the above-referenced problems and others.